1. Field of the Invention
The present invention relates to the formation of graphene-based devices.
2. Discussion of the Background
Graphene is a flat monolayer of carbon atoms tightly packed into a two-dimensional honeycomb lattice that resembles chicken wire. It is the building block for graphite material. Graphene has attracted great attention for electronic applications due to its extremely high mobility (e.g., over 200,000 cm2/V·s) and some unique properties (e.g., bandgap modulation by structure). However, the fabrication methods for graphene devices are still quite rudimentary.
Graphene has some unique material properties that make it very attractive for electronic applications. Its mobility has been demonstrated to be over 200,000 cm2/V·s in experiments. Unlike the high mobility in some un-doped bulk semiconductors, graphene mobility remains high even at high carrier concentration. Carrier transport in graphene can be ballistic on the micrometer scale at room temperature. Similar to carbon nanotubes, graphene has ambipolar electric field effect, i.e., it can be electrically doped to become n-type or p-type depending on the gate voltage. The badgap of semiconducting graphene can be modulated by its structure, e.g., the width of a graphene stripe. Graphene also has superior thermal conductivity that can enable some on-chip thermal management functions. As a natural two-dimensional system with planar form, graphene is easier to pattern than carbon nanotubes. Also, graphene can be potentially made with very low cost.
The first few-layer graphene was prepared by mechanical exfoliation of highly-oriented graphite. (See, e.g., K. S. Novoselov, et al, “Electric Field Effect in Atomically Thin Carbon Films”, Science 306, 666 (2004).) Even a graphene monolayer can be prepared with this method; however, such a method is unsuitable for production. Another method of growing graphene is the thermal decomposition of SiC where Si atoms are removed and C atoms left behind form graphene-like structures in the surface layers. (See, e.g., C. Berger, et al., “Ultrathin Epitaxial Graphite: 2D Electron Gas Properties and a Route toward Graphene-based Nanoelectronics”, J. Phys. Chem. B 108, 19912 (2004).) A drawback of this method is the high temperature (˜1400° C.) in this process. More methods are being developed including chemical synthesis. (See, e.g., S. Gilje, et al., “A Chemical Route to Graphene for Device Applications”, Nano Lett. 7, 3394 (2007).) It is expected that some low-temperature material preparation methods suitable for production will be developed for graphene.
Current device fabrication methods all start with the preparation of a graphene layer followed by the patterning of device structures. For example, FIG. 5 depicts a graphene-based field effect transistor (FET) structure, which is fabricated and structured according to a related art process. Such a related art process for fabricating graphene devices includes providing a substrate 110 and then first growing or depositing a graphene layer on the substrate 110. Then, the process includes patterning the device structures by etching the graphene layer to form graphene 112, and depositing source and drain regions 114, a gate dielectric 116 and a gate metal 118. The process is limited by the difficulty of handling of the graphene layer. Since graphene has to be made extremely small to exhibit some unique physical properties, the patterning and etching of graphene into extremely small sizes presents a challenge using current fabrication methods.
Thus, a need exists for a process for forming graphene-based devices that simplifies the fabrication process of such devices and provide better process control.